This invention relates to an aqueous coating composition and in particular to an aqueous coating composition containing a transparent iron oxide pigment dispersion formed with a synergistic mixture of acrylic copolymer pigment dispersants that has improved transparency and reduced haze.
The use of transparent iron oxide pigments in coating compositions used in particular for exterior finishes for automobiles and trucks to provide the finish with high transparency, glamour and aesthetic quality is well known. High film transparency is particularly important for automotive coatings containing metallic flake pigments, such as aluminum flake, as it is needed to impart high metallic luster to the finish. To obtain films of high transparency, the pigment it particles must be fully deflocculated and uniformly dispersed in the coating composition. Pigment dispersions stabilized with copolymer dispersants are generally used to disperse and maintain, in a dispersed state, transparent iron oxide pigments in coating compositions. Copolymer dispersants used to form such pigment dispersions are well known in the art and typically include random, block and graft copolymers that have a pigment adsorbing segment that is attracted to the pigment surface and a stabilizing segment that extends into the surrounding solvent medium to provide entropic or steric stabilization of the pigment particles against flocculation and keep the pigments dispersed.
There are relatively few problems with the addition of transparent iron oxide pigment dispersions to solvent based coating compositions, but in waterborne compositions, stabilization of transparent iron oxide pigments is a difficult task. In typical aqueous coating compositions, other particulate materials such as latex, other pigments, electrolytes, etc. compete for the iron oxide surface and have a tendency to displace the dispersing resin from the pigment surfaces, which causes the pigments to flocculate or cluster together and jeopardizes the pigment stability necessary to obtain high transparency and low haze in the final coating.
The stabilization of transparent iron oxide in waterborne systems therefore depends, at least in part, on the ability to have the dispersing polymer favorably associated with the pigment surface. The existing polymeric dispersing resins used to stabilize transparent iron oxide pigments in aqueous coatings compositions do not adequately meet this criteria. Such resins, which include acrylic copolymers having a hydrophobic adsorbing segment containing acid groups for enhanced interaction with the pigment surface and a hydrophilic stabilizing segment for aqueous solubility, as for example, as taught in U.S. Pat. No. 5,231,131 to Chu et al issued Jul. 27, 1993, incorporated by reference herein, produce stable aqueous transparent iron oxide pigment dispersions that have very good shelf life, but when the dispersions are added to aqueous coating compositions, they tend to undergo pigment flocculation and phase separation due to competing effects described above which result in poor paint stability, high haze, high viscosity, and ultimately coatings having diminished transparency and color strength.
Phosphated acrylic copolymer dispersants are also known in the art, as for example, as taught in U.S. Pat. No. 5,502,113 to Antonelli et al issued Mar. 26, 1996 and U.S. Pat. No. 5,530,070 to Antonelli et al issued Jun. 25, 1996, both incorporated by reference herein, and have been used to form aqueous pigment dispersions and coating compositions containing metallic flake, e.g., aluminum flake, pigment. Although these dispersants are effective in stabilizing metallic flake pigments in waterborne systems, they are not sufficient when used alone in stabilizing transparent iron oxide pigments.
Therefore, there is a need to improve the performance of aqueous coating compositions containing transparent iron oxide pigments, and in particular to find new pigment dispersions for use therein containing dispersing resins that have greater affinity for the transparent iron oxide pigment surface and provide more effective pigment stabilization, and ultimately improve the transparency of coatings formed therefrom and come close to the performance of solvent borne systems.
The present invention teaches a method of using a phosphated acrylic copolymer dispersant, as an auxiliary dispersant, in combination with a traditional transparent iron oxide dispersant to improve the transparency of aqueous coating compositions containing transparent iron oxide pigments. The present invention also teaches a method of using the auxiliary phosphated dispersant as an anchoring polymer which provides anchoring sites for copolymer dispersants with alkyl amino groups to provide a venue for stronger adsorption of polymeric dispersants to the pigment surface and greater coating transparency.
The present invention relates to an improved aqueous coating composition containing
(a) an aqueous carrier medium:
(b) transparent iron oxide pigment particles stabilized by
(c) a synergistic mixture of acrylic copolymer dispersants consisting essentially of
(i) an acrylic copolymer dispersant having a hydrophilic stabilizing segment and acid groups in a hydrophobic pigment adsorbing segment
(ii) an acrylic copolymer dispersant having a hydrophilic stabilizing segment and phosphate groups in a hydrophobic pigment adsorbing segment and
(iii) optional acrylic copolymer having a hydrophilic stabilizing segment and alkyl amino groups in a hydrophobic pigment adsorbing segment;
wherein the pigment and dispersants are present in a dispersant (i) to pigment weight ratio, D/P, of about 0.1/1 to 1/1, a dispersant (ii) to pigment weight ratio, D/P, of about 0.05/1 to 0.33/1, and a dispersant (iii) to pigment weight ratio, D/P, if present, of about 0.05/1 to 0.2/1;
(d) a film forming polymeric binder; and
(e) a crosslinking agent for the binder.
Pigment dispersions of the forgoing acrylic copolymer combination and process for forming the dispersions also are part of this invention.